Horizontal shaft impact crushers (HSI-crushers) are utilized in many applications for crushing hard material, such as pieces of rock, ore etc. A HSI-crusher has an impeller that is made to rotate around a horizontal axis. Pieces of rock are fed towards the impeller and are struck by beater elements mounted on the impeller. The pieces of rock are disintegrated by being struck by the beater elements, and are accelerated and thrown against breaker plates, often referred to as curtains, against which further disintegration occurs. The action of the impeller thus causes the material fed to the horizontal shaft impact crusher to move freely in a crushing chamber and to be crushed upon impact against the beater elements, against the curtains, and against other pieces of material moving around at high speed in the crushing chamber.
U.S. Pat. No. 6,189,820 discloses a HSI-crusher having a crushing chamber in which material supplied to the crusher can be crushed. On some occasions it is difficult to adjust the curtains to obtain the desired particle size in the crushed product leaving the crusher.